This disclosure pertains to microlithography (transfer of a pattern to a sensitive substrate), especially as performed using a charged particle beam. Microlithography is a key technology used in the fabrication of microelectronic devices such as integrated circuits, displays, and micromachines. More specifically, the disclosure pertains to charged-particle-beam (CPB) microlithography performed using a pattern-defining segmented reticle on which the pattern is divided into multiple subfields each defining a respective portion of the pattern, and to methods by which respective images of the subfields are transferred from the reticle to the substrate.
With the relentless drive to progressively smaller feature sizes (now less than 0.10 xcexcm) the pattern-resolution limitations of optical microlithography have become a major limitation. To solve this problem, considerable effort currently is being expended to develop a practical xe2x80x9cnext generationxe2x80x9d microlithography technology. A major effort to such end involves using a charged particle beam (e.g., an electron beam) as the lithographic energy beam. Charged-particle-beam (CPB) microlithography is expected to produce substantially better pattern resolution for reasons similar to the reasons for which electron microscopy yields better image resolution than optical microscopy.
Current CPB microlithography technology does not yet embody a solution to the problem of projecting an entire pattern in one shot from the reticle to the substrate. Rather, the pattern is divided into individual exposure units usually termed xe2x80x9csubfieldsxe2x80x9d that are defined on a xe2x80x9csegmentedxe2x80x9d reticle and exposed in a prescribed order subfield-by-subfield. This exposure scheme is termed xe2x80x9cdivided-reticle pattern transfer.xe2x80x9d As can be surmised, the optical field of CPB optics required to transfer a single subfield is much smaller than otherwise would be required to transfer the entire pattern in one shot. During transfer of each subfield, the respective subfield image is formed on the substrate in a manner such that, when exposure is complete, the subfield images collectively form the entire contiguous pattern. The subfields typically are arrayed on the reticle in rows and columns, wherein each row has a length substantially equal to the diameter of the optical field of the CPB optical system. During exposure of a row of subfields, the charged particle beam is deflected laterally as required to transfer the subfields in the row in sequential order. In progressing from one row to the next, the reticle and substrate typically are mechanically scanned in opposite lateral directions.
Whenever a pattern is being transferred from a segmented reticle, it is desirable to achieve the greatest possible pattern-transfer accuracy. To such end, the subfield images are formed at respective locations on the substrate that desirably result in proper xe2x80x9cstitchingxe2x80x9d together of the individual subfield images. Such stitching (i.e., positioning of respective subfield images) must be performed extremely accurately. However, in a practical sense, situations do arise in which adjacent subfield images are slightly misaligned with each other, resulting in a corresponding xe2x80x9cstitchingxe2x80x9d error. Stitching errors can be manifest, for example, as shorts between adjacent wiring traces caused by overlap of one wiring trace over another, and as breaks in wiring traces that should be contiguous with each other. With increases in the number of intersections of adjacent subfields as imaged on the substrate and/or increases in the number of patterns elements per subfield that are joined together, the number of such faults tends to increase commensurately. In addition, in conventional segmented reticles, subfield boundaries sometimes are situated between the source and gate or between the gate and drain of a transistor of the pattern. In such a situation, a stitching error at the subfield boundary can cause the respective transistor simply not to function or to function improperly.
Japan Kxc3x4kai Patent Document No. Hei 9-97759 discloses a conventional method by which a pattern as defined on the reticle is divided into subfields. A plotter diagram of the pattern is evaluated so as to place subfield-division lines in respective locations that reduce intersections of the lines with pattern elements as much as possible. Unfortunately, in this method, pattern evaluation requires that an operator handle very large amounts of data; hence, data processing requires a long time to complete.
Whenever a pattern is divided to form a segmented reticle, situations frequently arise in which certain pattern elements cannot be defined entirely in a single subfield. For example, certain pattern elements inevitably result in the presence of xe2x80x9cisland,xe2x80x9d xe2x80x9cdonut,xe2x80x9d xe2x80x9cpeninsular,xe2x80x9d or other reticle features that are not self-supporting. In such instances, at least two complementary subfields must be used to define the respective pattern portion. U.S. Pat. No. 5,166,888 discloses a conventional method for dividing a pattern portion into complementary subfields. In the disclosed method, a stability value is assigned to each inside corner of the pattern element. The stability value is a function of the length of the perimeter of all edges between adjacent outside corners of the pattern element and the length of the shortest distance between the adjacent outside corners. Based on such data, pattern elements having a sub-threshold stability value are divided into multiple rectangular sections. The resulting sections are distributed between two complementary subfields. Unfortunately, this method tends to result in the element being divided up more than actually necessary, which results in a higher than acceptable incidence of stitching errors between adjacent sections of the element as transferred to the substrate.
Also, the methods summarized above are directed to, and thus have been applied only to, abstract artificial patterns rather than actual LSI patterns. Hence, there is a need for pattern-element division methods that are more applicable to actual LSI patterns.
In view of the shortcomings of conventional methods as summarized above, this invention provides, inter alia, transfer-exposure methods for performing division of pattern elements that are more similar to actual LSI patterns, and that provide improved pattern-layering accuracy and reduced stitching errors.
To such end, and according to a first aspect of the invention, methods are provided for dividing a pattern to be defined on a segmented reticle for use in charged-particle-beam (CPB) microlithography. The pattern is divided into multiple subfields each defining a respective portion of the pattern. In an embodiment of such a method, the reticle pattern is initially divided using initial subfield-boundary lines that are spaced apart from one another according to a subfield dimension determined in advance. Then, the initial subfield-boundary lines are corrected to produce corrected subfield-boundary lines so as to correct pattern-element stitching when the pattern is transfer-exposed to the sensitive substrate.
If the pattern elements as defined on the reticle are arranged at certain respective pitches in the X direction and Y direction, then the initial subfield-boundary lines can be established at respective integer multiples of the respective pitches without, thereby, defining subfields that are larger than a maximum subfield size that can be exposed without excessive aberrations. In addition to dividing the reticle into subfields each defining a respective portion of the pattern, certain subfields can be divided as required into respective complementary subfields so as to avoid the xe2x80x9cdonutxe2x80x9d problem and/or the xe2x80x9cpeninsulaxe2x80x9d problem. For example, in the reticle subfields that are divided into respective complementary subfields, each respective reticle subfield can be divided by at least one line extending in the X or Y direction at an interval of xc2xc of the respective pitch or an integer multiple of xc2xc of the respective pitch.
Typically, the reticle pattern has a minimum linewidth, wherein the respective integer multiples (between adjacent initial subfield-boundary lines) desirably are within a range of 50 to 500 times the minimum linewidth.
If the reticle pattern extends in X and Y directions along at least one of which the reticle pattern exhibits a cyclical period repeat, then the initial division step can include placing the initial subfield-boundary lines at an integer multiple of half the period repeat.
Typically, pattern elements in the reticle pattern include one or more respective xe2x80x9csignificant pointsxe2x80x9d as defined herein. With respect to such pattern elements, if the respective significant points are located within a predetermined distance from the respective initial subfield-boundary lines, then the initial subfield-boundary lines desirably are corrected by shifting the respective lines so as to intersect the significant points. Furthermore, an initial subfield-boundary line can be shifted to intersect a significant point of a respective adjacent pattern element if the respective significant point defines an interior angle of at least 180xc2x0. If the respective significant point does not define an interior angle of at least 180xc2x0, then the initial subfield-boundary line can be shifted to intersect a significant point of a respective adjacent pattern element if the respective significant point defines an interior angle of less than 180xc2x0.
If the reticle pattern extends in X and Y directions along at least one of which the reticle pattern exhibits a regular cyclical repeat having a respective pitch, then, in the X or Y direction in which the reticle pattern exhibits the regular cyclical repeat, the corrected subfield-boundary line can extend through significant points located, relative to the respective initial subfield-boundary line, within xc2xd the respective pitch.
Desirably, in the correcting step, the corrected subfield-boundary lines are configured so as to intersect as few adjacent pattern elements as possible while intersecting the respective significant points.
In the initial-division step the initial subfield-boundary lines can be configured to extend in the X and Y directions so as to extend along pattern elements extending in a similar manner. In such an instance, in the correcting step each corrected subfield line can be configured to extend, within a respective pattern element, in at least one of the X and Y directions as the corrected subfield line connects together respective significant points of the respective pattern element. Additionally, in the correcting step each corrected subfield line can be configured to extend, outside a respective pattern element, parallel to at least one of the X and Y directions or at a slope relative to at least one of the X and Y directions. The slope can be within a range of xc2x145xc2x0 or xc2x1135xc2x0 relative to the X or Y direction.
In any of the method variations summarized above, if the significant points are located within 20 times the minimum linewidth of the respective initial subfield-boundary lines, then the initial subfield-boundary lines can be corrected by shifting the respective lines so as to intersect the significant points.
In any of the method variations summarized above, if the significant points are located within a distance, determined in advance, no greater than 20 times the minimum linewidth of the respective initial subfield-boundary lines, then the initial subfield-boundary lines can be corrected by shifting the respective lines so as to intersect the significant points.
In any of the method variations summarized above, after the reticle pattern has been initially divided (using the initial subfield-boundary lines), calculations for determining the respective locations of the xe2x80x9ccorrectedxe2x80x9d subfield-boundary lines need only involve data pertaining to the vicinity of the initial subfield-boundary lines. Consequently, data-processing and calculation time is substantially reduced compared to conventional methods. In addition, human intervention is not necessary for dividing the pattern, and the pattern can be divided completely using automated equipment.
Furthermore, the pattern is not subdivided more than necessary, thereby substantially reducing stitching errors between adjacent subfields as projected onto the substrate, or between complementary subfields as projected onto the substrate. Moreover, the methods disclosed herein are readily applicable to actual LSI patterns. xe2x80x9cSignificant pointsxe2x80x9d are defined later below. In any event, the significant points are respective loci at which the consequences of a joining error (generally termed a xe2x80x9cstitching errorxe2x80x9d) are relatively minor. Thus, by dividing the pattern along lines extending through significant points of adjacent pattern elements, it is possible to improve layering accuracy between successive pattern layers applied to a substrate.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.